The present invention relates to communication systems and in particular to modulation of information symbols in communication systems.
In usual communication systems, for example wireless communication systems, information symbols are coded and modulated at the sender, transmitted on the medium and demodulated and decoded at the receiver.
Depending on the communication system, demodulation can be performed in a coherent or a non-cohere rent manner.
Coherent detection requires equalization and channel response estimation so that the effects of phase and magnitude distortion caused by the communication channel can be compensated for with matched filters. Coherent demodulation brings prohibitive complexity and poor robustness.
Non-coherent detection, on the contrary, is based on the fact that decision for received symbols can be made without compensating for the phase distortion of the received signal. Non-coherent detection at the receiver is preferred to coherent detection because of the relative simplicity at the receiver.
As is well known from those skilled in the art, orthogonal modulation are conventionally used since specially good appropriate for non-coherent detection at the receiver. However, signals modulated by an orthogonal modulation are very bandwidth consuming. A compromise often consists in reducing the bandwidth by using non-orthogonal modulations resulting however in a performance degradation in the non-coherent detector since the transmitted signals are correlated.
The design of multidimensional constellations for non-coherent, non-orthogonal modulations has proved to be a good way to compensate the performance degradation at the non-coherent receiver.
In multiple users communication networks, there are several ways for users to send information through the communication channel. For both Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) techniques, the channel is basically partitioned into equal independent and non-overlapping single user subchannels. Each user is respectively assigned a particular time slot within each frame or a frequency subchannel. These techniques are frequently used in data and digital voice transmission. However both methods tend to be inefficient when users transmit bursty information. In this particular case, an alternative is to allow more than one user to share a channel or subchannel by use of direct-sequence spread spectrum signals. In this technique, called Code Division Multiple Access (CDMA), each user is assigned a unique code sequence and spreads the information signal across the assigned frequency band. Thus, signals from the various users are separated at the receiver by cross-correlation of the received signal with each of the possible user signature sequences. CDMA is a promising technique for radio access in future cellular mobile and personal communication systems. It offers some attractive features compared to TDMA or FDMA such as the potential for high radio capacity, soft handover, simplified frequency planning, etc.
In non-coherent TDMA and FDMA systems, Q-ary Frequency Shift Keying (FSK) is used and non-coherent detection is made with Q orthogonal FSK signals having a tone spacing equal to the inverse of the symbol period. Reducing the tone spacing of FSK signals leads to non-orthogonal modulations that can be advantageously used to reduce the bandwidth of the modulated signals.
In non-coherent CDMA systems, the modulation is preferably chosen to be a family of orthogonal spreading sequences, e.g. Walsh-Hadamard. However, the number of strictly orthogonal spreading sequences is limited for a given spreading sequence length (the spectral efficiency of orthogonal spreading sequences is poor). Non-orthogonal spreading sequences are then advantageously used to enhance the system capacity. They are mainly characterised by their cross-correlation, which is equivalent to the tone non-orthogonality in the FSK case.
A known solution is to replace the Walsh-Hadamard set by a new non-orthogonal set. When this kind of sets cannot be used, one can think of considering several masked versions of an initially orthogonal set (used for instance in IS-95 systems). An alternative is to use well known families of PN-like sequences (e.g. Gold or Kasami sequences).
U.S. Pat. No. 5,938,787 describes a method of encoding information symbols according to a concatenation of an error correction code and a non-orthogonal modulation code where the non-orthogonal modulation code is obtained by a translation of a set of orthogonal code vectors according to a predetermined translation. This modulation set contains spreading sequences with binary chip values. This fact implies a strong limitation on the number of spreading sequences that could be generated. Indeed, when considering spreading sequences of N chips length, the ultimate upper limit on the number of spreading sequences is 2N. A large number of these 2N spreading sequences is not usable since a limit for the cross-correlation value between the spreading sequences should be ensured.
In “Complex spreading sequences with a wide range of correlation properties” (I. Opperman and B. Vucetic, IEEE transaction on communications, VOL. 45 NO. 3, March 1997, pages 365 to 375), a method for building sets of complex spreading sequences with good correlation properties is presented. For ensuring good correlation properties, the number of spreading sequences M constituting the alphabet is chosen smaller than the spreading sequence length N.
All above described modulation sets have a low spectral efficiency. The spectral efficiency can defined be as log2(M)/N and represents the number of bits to map a sequence divided by the sequence length. This quantity is given in bit/s/Hz. For the modulation sets described above, the spectral efficiency is <<1 (very smaller than one).
The object of the present invention is to provide a method for transmitting coded information symbols in a communication network by using a non-orthogonal modulation code having a large spectral efficiency.
Another object of the invention is to provide a non-orthogonal modulator for performing the above mentioned method.
Another object of the present invention is to provide non-orthogonal modulation codes families to be used by the non-orthogonal modulator.